Refugium w/DSB ? for 125g with 40g sump

I currently have a 125g that has a 40g breeder for the sump that I just switched to a few weeks ago from a 29g. The new sump has 3 chambers since I wanted to add a refugium... Thinking I was doing the right thing I added 3" of sand to the center and LR rubble. I have done loads of research and some advocate for sand and some advocate for BB. My question is since the new sump has only been up and running for a month can I safely switch to BB? My old sump was BB but only had 2 chambers, 1 for the skimmer and 1 for the return... I plan on keeping LR and Cheato in the fuge... So do I need/want sand in there? Or can I siphon in out and slowly add it to the display which could use a little more in the front?

Background info:

125g 4 years old, all sand replaced last August due to a move from Texas (which is why I still need more in the front)